On Sat, Sep 3, 2016 at 8:28 AM, Stephen J. Turnbull <turnbull.stephen.fw@u.tsukuba.ac.jp> wrote:

Guido van Rossum writes:

...

But that's not what type comments mean! They don't annotate the expression. They annotate the variable.

In PEP 484. But syntactically, AFAICS in an initialization that's a distinction without a difference. It would be perfectly possible to write a checker that allows

if cond: x: str = "a string, what else?" else: x: int = 1

and infers the union, and even infers the types from the expressions in

if cond: x = "a string, what else?" else: x = 1

Once you have that, then the only things that are really being typed are the initializer expressions.

I don't understand why some people seem to think that is a *good* typechecker (if that's it's normal mode of operation). But as a tool to identify the types that untyped (or ambiguously-typed) code actually uses, review them, and help produce a precisely typed version, it seems perfectly plausible to me.

That's is why we're not restricting this syntactically. I've seen a fair amount of untyped code that reuses the same variable name (in the same scope, often in unit tests), but usually it was serially, e.g. target = 42 assert fooi(target) == '42' target = 3.14 assert foof(target) == '3.14' target = 'x' assert foos(target) == "'x'" -- --Guido van Rossum (python.org/~guido)

On Sat, Sep 3, 2016 at 8:28 AM, Stephen J. Turnbull <turnbull.stephen.fw@u.tsukuba.ac.jp> wrote:

Guido van Rossum writes:

...

But that's not what type comments mean! They don't annotate the expression. They annotate the variable.

In PEP 484. But syntactically, AFAICS in an initialization that's a distinction without a difference. It would be perfectly possible to write a checker that allows

if cond: x: str = "a string, what else?" else: x: int = 1

and infers the union, and even infers the types from the expressions in

if cond: x = "a string, what else?" else: x = 1

Once you have that, then the only things that are really being typed are the initializer expressions.

I don't understand why some people seem to think that is a *good* typechecker (if that's it's normal mode of operation). But as a tool to identify the types that untyped (or ambiguously-typed) code actually uses, review them, and help produce a precisely typed version, it seems perfectly plausible to me.

That's is why we're not restricting this syntactically. I've seen a fair amount of untyped code that reuses the same variable name (in the same scope, often in unit tests), but usually it was serially, e.g. target = 42 assert fooi(target) == '42' target = 3.14 assert foof(target) == '3.14' target = 'x' assert foos(target) == "'x'" -- --Guido van Rossum (python.org/~guido)